Tính nhanh: 4/2.4 + 4/4.6 + 4/6.8 + 4/8.10 +.... + 4/18.20
Giải thích
Đặt A=42.4+44.6+46.8+48.10...+418.20
=2.22.4+24.6+26.8+28.10...+218.20=2.12−14+14−16+16−18+18−110...+118−120=2.12−120=1−110=910
Vậy A=910
Đặt A=42.4+44.6+46.8+48.10...+418.20
=2.22.4+24.6+26.8+28.10...+218.20=2.12−14+14−16+16−18+18−110...+118−120=2.12−120=1−110=910
Vậy A=910